I had the same problem-- Interestingly it coincided with a loud, unusual
whirring that got worse every day:  The coolant pump!  It was giving up
the ghost and increasing amounts of precious coolant with it.

The way to tell if your pump is dying is:
-check the tension on the belt that runs the pump:  Should be tight-tight!
(technical term)
-After the van has been sitting overnight or longer:  Give yourself a few
extra minutes _before_ starting the engine; do a
preliminary check underneath to locate the spot where the pump
attaches to the engine.  Dry or wet?  Any drops on the oil filter?
-Start the engine and run back to look underneath again at the pump and
oil filter (some would warn you to wear safety glasses! I guess I just did
too.)  Please make sure the brakes are on tight!!  (having kids has made
me absolutely nutty about safety).
-As noted in a previous post, if the pump is lunched, it may leak coolant
as soon as it starts pumping it, dribbling down over the oil filter.
-Replacing the pump is a whole other archive, the likes of which I'm sure
exists somewhere.  There are brand-new pumps and re-conditioned ones, and
installing it requires flushing the coolant system.  Yet another archive
unto itself!

I was quite chuffed to have spotted this problem myself... I quite
promptly pointed it out to my mechanic!  I would love to start DIYing
(Do-It-Yourself) but with two small kids at home with me all day I have
little time for such fun.  I am asking for a torque-wrench and
jack-stands for Christmas, though.

Hoping this adds insight,

Marcus Elia
'84 Westy (Dela)